With regard to a vehicular air-conditioning control device disclosed in JP-A-10-287123, a first thermistor for detecting temperature of a heat generating portion of an electric heater and a second thermistor for detecting temperature of water are disposed in a hot-water tank. When it is determined that the temperature difference between detected temperatures of the first thermistor and the second thermistor exceeds a predetermined level, it is judged that an abnormal condition is encountered. Thus, electric power supply to the electric heater is interrupted.
With regard to a cleaning device for bath hot water disclosed in JP-A-7-35407, hot water in a bath tub is forcedly circulated by a circulation pump and is filtered in a filter tank. Also, the heat of the water is maintained in a thermal insulating heater and activated in an activation tank. A first hot-water temperature sensor is disposed upstream of the thermal insulating heater and a second hot-water temperature sensor is disposed downstream of the thermal insulating heater. When it is determined that the temperature difference between detected temperatures of the first and the second sensors exceeds a predetermined level, it is judged as a shortage of water. Thus, operation of the cleaning device is discontinued.
In a water heating device having a mechanical flow sensor, resistance is caused to a circulation of water. Further, it is required to tightly seal connecting portions of the flow sensor to prevent leaks of the water. Therefore, it is difficult to decrease manufacturing costs of the hot water generating device. Also in a water heating device having an electric flow sensor, it is difficult to decrease the manufacturing costs because an electric circuit is complicated.
Further, in a water heating device that detects an electric current supply to a water pump, a shunt resistor for detecting an electric current is required. This results in increases in size of an electronic control unit and the manufacturing costs. Further, in a water heating device that detects rotation speed of a water pump, a sensor for detecting the rotation speed is required. This results in increases in the water heating device and the manufacturing costs.